1. Field of the Invention
The present invention relates to a process for producing a membrane-electrode assembly for solid polymer electrolyte fuel cells. Particularly, it relates to a process for producing a membrane-electrode assembly for solid polymer electrolyte fuel cells having its reaction efficiency improved by dissolving non-uniformity of the current density in the plane of the membrane-electrode assembly by a simple method.
2. Discussion of the Background
Fuel cells are expected to be widely used in the future since their power generation efficiency is high, and their load to the environment is light. Particularly solid polymer fuel cells are expected to be widely spread for movable bodies such as automobiles, or as a distributed power generation system or a cogeneration system for home use, since their power density is high and their operating temperature is low, whereby downsizing and cost cutting are easy as compared with other fuel cells.
In general, as illustrated in the sectional view of FIG. 4, a membrane-electrode assembly 101 for solid polymer electrolyte fuel cells comprises a solid polymer membrane 103 comprising an ion exchanger polymer, catalyst layers 105a and 105b of an anode and a cathode, respectively, bonded to both sides of the solid polymer membrane 103, and e.g. carbon paper or carbon cloth as gas diffusion layers 107a and 107b disposed outside the catalyst layers.
Outside the gas diffusion layers 107a and 107b, an electrically conductive separator 109 is disposed. On the separator 109, gas flow paths 111a and 111b, which face the gas diffusion layers 107a and 107b, are formed. The gas flow path has, specifically, various modes such as a series groove 111c and a parallel groove 111d, which extend from the inlet 109a to the exit 109b, as shown in FIGS. 5 and 6.
As described above, the membrane-electrode assembly 101 is formed by bonding the catalyst layers 105a and 105b containing a noble metal on both sides of the polymer electrolyte membrane 103. The catalyst layers 105a and 105b are formed by a method of directly coating the polymer electrolyte membrane 103 with an ink for formation of a catalyst layer, containing as the main component a dispersion of a catalyst-supported carbon and a solid polymer electrolyte resin (such as a perfluorocarbon polymer having sulfonic acid groups) or a method wherein a substrate is preliminarily coated with the above ink to form catalyst layers 105a and 105b in the form of a sheet, which are bonded to the polymer electrolyte membrane 103 by means of e.g. hot pressing.
As a specific method of preparing the catalyst layers 105a and 105b on the substrate, a method of forming the layers on the substrate 125 for coating by using a die 121 shown in FIG. 7 may be mentioned. In this method, the substrate 125 for coating is coated, for example, with the above-described ink for formation of a catalyst layer to form the catalyst layer 105a as one of the catalyst layers. Further, a dispersion of an ion exchange polymer may be cast on the catalyst layer 105a by using the die 121 to form the polymer electrolyte membrane 103. Otherwise, the polymer electrolyte membrane 103 may be formed by a cast film forming in advance, and the catalyst layer 105a is formed thereon.
The membrane-electrode assembly 101 for solid polymer electrolyte fuel cells thus constituted makes a fuel gas and an oxidant gas pass through the gas flow paths 111a and 111b, respectively, of the separator 109, and at the same time, transmits electricity to the outside through the gas diffusion layers 107a and 107b, and with which electric energy can be taken out.
In the membrane-electrode assembly 101, a cell reaction takes place by the gas supplied from the separator 109. The supplied gas is consumed by the cell reaction, and a reaction product such as water is formed, and accordingly the reaction gas composition, a moistening condition of the gas, etc. change along the gas flow path and as a result, the reaction conditions also change along the gas flow path. Due to this change of conditions, the current density becomes non-uniform in the plane of the membrane-electrode assembly 101, which is one cause of decrease in cell performance.
To overcome the above problems, in order to secure a uniform reaction efficiency in the entire plane of the membrane-electrode assembly 101, it has been proposed to change the amount of a catalyst from the inlet 109a toward the exit 109b of the gas flow path (JP-A-3-245463, JP-A-2000-149959). Specifically, the coating amount of the catalyst is changed utilizing the concentration gradient depending upon the distance in a spray coating or by utilizing the concentration gradient depending upon the number of coating in the screen printing.
However, a high level of control a coating thickness is required to change the coating amount for the catalyst layers 105a and 105b of the membrane-electrode assembly 101 by spray coating. Further, in the case of coating by the screen printing, a complicated coating process and a gradual change of the coating amount are inevitable.
Under these circumstances, it is an object of the present invention to provide a process for producing a membrane-electrode assembly for solid polymer electrolyte fuel cells having its reaction efficiency improved, by dissolving non-uniformity in the current density in the plane of the membrane-electrode assembly by a simple method.